1. Field of the Invention
The present invention relates to optical communications and, in particular, to the generation of high-speed digital optical signals.
2. Description of the Related Art
The principles of modulating an optical beam to create a high-speed (i.e., high data rate) digital optical signal are well known in the art. One traditional method, as shown in FIG. 1, involves electronically multiplexing a pair of low-speed electrical signals 101 having the same data rate via a time-division multiplexer 103 to create a high-speed time-division multiplexed electrical signal 105 of desired data rate. The low-speed electrical signals 101, even though labeled as low-speed electrical signals, in a typical case, range from 10 Gb/s to 20 Gb/s. The high-speed time-division multiplexed electrical signal 105 is then input to an optical modulator 107 which uses the high-speed time-division multiplexed electrical signal 105 to modulate a continous-wave (cw) optical beam 109 generated by a laser diode. The output from the optical modulator 107 is a high-speed digital optical signal 111 with the data rate of the time-division multiplexed electrical signal 105, but having digital optical characteristics.
The prior art scheme shown in FIG. 1 results in the desired high-speed digital optical signal but this scheme is limited in scope. As the desired data rate of the time-division multiplexed signal increases, it becomes increasingly difficult to multiplex low-speed electrical signals 101 via commercially available time-division multiplexers 103. For example, commercial time-division multiplexers 103 cannot generate signals having a data rate higher than 40 Gb/s.
Another prior art technique, as shown in FIG. 2, is Optical Time-Division Multiplexing (OTDM). This technique involves receiving two low-speed electrical signals 201, inputting each of them to an optical RZ (Return-to-Zero) modulator 202. Each optical RZ modulator 202 uses a low-speed electrical signal 201 to modulate an optical beam 209 and generates a modulated optical signal 203. Then the two modulated optical signals 203 are optically combined (i.e., multiplexed optically) via an optical multiplexer 205 to generate a high-speed digital optical signal 211. The scheme in FIG. 2 varies from the scheme in FIG. 1 by the fact that low-speed electrical signals are first used to generate modulated optical signals that are then optically multiplexed in FIG. 2 rather than first electrically multiplexed and then used to generate a modulated optical signal as in FIG. 1.
The OTDM technique of FIG. 2 is an effective way to create a desired digital optical signal, but is very expensive to integrate in a commercial device. This technique also generates an undesirable coherent beat noise in the resulting high-speed digital optical signal 211.
A new and improved apparatus and method for generating high-speed digital optical signals are provided. In accordance with the principles of the present invention, two low-speed electrical signals are first input to two electrical Return-to-Zero (RZ) converters to generate RZ electrical signals. After one of the RZ signals is inverted, the RZ electrical signals are then input to a dual-electrode optical modulator, e.g., a Mach-Zehnder interferometer, to generate the desired high-speed digital optical signal.
The dual-electrode optical modulator of the present invention has a pair of electrodes driven with a pair of differential signals. This dual-electrode optical modulator has multiplexing as well as optical modulating capabilities. As such, the optical modulator of the present invention replaces the separate multiplexer and modulator blocks of the prior art. Within the dual-electrode optical modulator, each electrode is coupled to receive a different low-speed RZ electrical signal. The optical modulator also receives an optical beam from a traditional light source, e.g., a laser diode. As the electrodes of the dual-electrode modulator optically modulate independently, they effectively combine the low-speed RZ electrical signals into one Non-Return-To-Zero (NRZ) optical signal by time-division multiplexing the RZ electrical signals as well as modulating the low-speed RZ electrical signals with an optical beam. The resulting output is a high-speed digital optical signal of the desired data rate.
The cost of generating high-speed digital optical signals in accordance with the principles of the present invention is relatively low. Unlike the prior art, the mechanism of the present invention is not limited to any particular data rate. In one embodiment, a method for generating digital optical signal is disclosed. This method comprises the steps of:
(a) converting a first electrical signal into a first Return-to-Zero (RZ) electrical signal;
(b) converting a second electrical signal into a second RZ electrical signal, wherein the second RZ electrical signal is inverted with respect to the first RZ electrical signal; and
(c) applying the first and second RZ electrical signals and an optical beam to a dual-electrode optical modulator to generate the digital optical signal.
In an alternative embodiment, an integrated circuit having a digital optical signal generator is disclosed. This digital signal generator comprises:
(a) a first electrical RZ converter configured to generate a first RZ electrical signal from a first electrical input signal;
(b) a second electrical RZ converter coupled to a signal inverter to generate a second RZ electrical signal from a second electrical input signal, wherein the second RZ electrical signal is inverted with respect to the first RZ electrical signal; and
(c) a dual-electrode optical modulator configured to receive the first RZ electrical signal and the inverted RZ electrical signal and configured to modulate the first RZ electrical signal and the inverted RZ electrical signal with an optical beam signal to generate a digital optical signal.